Seraphim

Very little is known about the Seraphim. Like if it is a single entity or a group of entities? One thing is for certain though - it/they are opposing the forces of Hell. Something which is quite in line with the roughly biblically inspired theme of the games. Seraphim are described as a high ranking(and powerful) caste of angels in the mythologies of Abrahamic religions. Naturally opposing demons. Little information can be extracted from a few entries in the codex. Among these is a picture(an etching from the "Book of Daeva") that contains an image of a/the Seraphim(upper left corner). This is interesting as it shows the Seraphim as some kind of "heavenly" figure watching over a battle of the Doom Slayer and some Night Sentinels. It´s not clear if this depicted battle takes place still in defense of Argent D'Nur or the early stages of the Hell Walker's campaign in Hell. Nevertheless it proves a connection between the Seraphim and the Doom Slayer and maybe also to the Night Sentinels and the world of Argent D'Nur, possibly even to the Wraiths(although that is a thin thread). The connection however makes sense as the Seraphim is/are also an enemy of Hell. But there are more fragments that shed some light:

Cited from the Slayer's Testament III:
"In his crusade, the seraphim bestowed upon him terrible power and speed, and with his might he crushed the obsidian pillars of the Blood Temples."

This might indicate that the Seraphim has interdimensional powers as he blesses the Doom Guy that is already in Hell. One must point out that there is no indicator to where the Seraphim naturally resides. In theory he/they could very well be also held captive in some place in hell but their power not completely restrained. Interestingly enough it´s nowhere seen or indicated that the Seraphim uses it´s/their powers directly against the demons(which might contradict the theory of them being captured in Hell). It´s also worth pointing out that the Seraphim did not bestow this power on him in the beginning as Argent D'Nur was still unconquered(could fuel the theory of imprisonment in Hell). That being all theories, there could very well be other explanations for these behaviour and decisions.

Written in the Slayer's Testament I:
"Burned by the embers of Armageddon, his soul blistered by the fires of Hell and tainted beyond ascension, he chose the path of perpetual torment."

With the whole biblical stuff looming around i like to ponder on the fact that it is stated that his soul is "beyond ascension". Later in the Testament it´s written that "He(Doom Guy) wore the crown of the Night Sentinels...". Could this indicate that there was some sort of ascension possible for a Night Sentinel(or anyone else)? If the Seraphim are a group of beings(which is a hypothesis) - did they recruit special powerful beings that made a stand against the forces of Hell, thus proving their worth(or "purity")? Demons are bolstering their ranks by corruption so it would make some sense that an opposing force has also some way to "recruit" new members into their ranks.

Synopsis
Apart from the fact that it/they oppose Hell and are quite powerful(although not using the power directly against demons), not much can be concluded about the Seraphim - apart from a few theories. Sadly there is not enough information available to further investigate a possible connection between Seraphim, Night Sentinels and Wraiths. Research will have to wait until the release of Doom Eternal or other enthusiasts of arcane lore reveal their findings.